Multi-functional magnetic mesoporous silica nanospheres (MMSNs), which were coated with poly(acrylic acid) (PM), have been synthesized using the atom transfer radical polymerization of tert-butyl acrylate on the surface of MMSNs followed by the hydrolysis of the grafted poly(tert-butyl acrylate) chains. The resulting MMSN-PAA nanocomposites exhibit negligible cytotoxicity toward HeLa and L02 cells. Magnetic resonance imaging (MRI) studies reveal that the nanocomposites can be effectively taken-up by the cancer cells. The anticancer drug doxorubicin hydrochloride (DOX) can be loaded into the nanocomposites and subsequently released in a sustained and pH-responsive way because of the presence of pH-sensitive polymer shells. The DOX-loaded nanocomposites exhibit notable cytotoxicity to HeLa cancer cells. These results demonstrate that the pH-responsive MMSN-PAA nanocomposites can be applied to biological systems for MRI and drug delivery. (C) 2012 Elsevier Ltd. All rights reserved.